2.1. The T Cell Antigen Receptor
T lymphocytes interact with antigens through the T cell antigen receptor (TCR) complex. The TCR is a clone-specific heterodimer on T cells, which recognizes its target antigen in association with a major histocompatiblity antigen. The TCR has been shown to be noncovalently associated with the CD3 complex. TCR is highly polymorphic between T cells of different specificities. Approximately 90 percent of peripheral blood T cells express a TCR consisting of an .alpha. polypeptide and a .beta. polypeptide. A small percentage of T cells have been shown to express a TCR consisting of a .gamma. polypeptide and a .delta. polypeptide. (Regarding TCR molecules, see Davis and Bjorkman, 1988, Nature 334:395-402; Marrack and Kappler, 1986, Sci. Amer. 254: 36; Meuer et al., 1984, Ann. Rev. Immunol. 2:23-50; Brenner et al., 1986, Nature 322:145-159; Krangel et al., 1987, Science 237:1051-1055; Hata et al., 1987, Science 238:678-682; Hochstenbach et al., 1988, J. Exp. Med. 168:761-776). The chains of the T cell antigen receptor of a T cell clone are each composed of a unique combination of domains designated variable (V), [diversity (D),] joining (J), and constant (C) (Siu et al., 1984, Cell 37:393; Yanagi et al., 1985, Proc. Natl. Acad. Sci. U.S.A. 82:3430). Hypervariable regions have been identified (Patten et al., 1984, Nature 312:40; Becker et al., 1985, Nature 317:430). In each T cell clone, the combination of V, D and J domains of both the alpha and the beta chains or of both the delta and gamma chains participates in antigen recognition in a manner which is uniquely characteristic of that T cell clone and defines a unique binding site, also known as the idiotype of the T cell clone. In contrast, the C domain does not participate in antigen binding.